Lower Manhattan
Wall Street is emptying out, for two main reasons: One, the city’s onerous taxes and regulations, and two, it’s not necessary for Wall Street to be on Wall Street any more, thanks to our technological era.
When will this happen to Washington, D.C.? The same reasons we don’t need a literal Wall Street are the same reasons we don’t really need a formal national capital. In fact, in this day and age of terrorism, it might be a good thing not to have one.
We already have a “shadow goverment” at the federal level just for the reasons you give.